What makes a diode so special? We know (?) that rectifier diodes are heavily optimized for low forward drop. Uf5408 on the other hand, have considerably more forward drop while it provides fast trr. Is it "safe" to use instead of 1N5408? Surely that trr is gonna have some benefit when it is on that bridge? Or the benefit does not outweigh the costs here?

There is also a follow up fast diode between bridge rectifier and the rectifier 400V-220uF capacitor: 5R06X with 25ns reverse recovery time(trr). I found uf5408 with 75ns trr in my hand I can't buy that to 220 packed 5r06x. So I have to cool it make a custom mount heatsink to its cylinder package?

I also found a more expensive 60ns trr TO220 packed MUR1560G diode but I don't think I should bother just for 15ns improvement, or the TO220 is really a must? I don't think these things really matter if I am not using the supply at full load for hours, am I right? What is the power dissipation typically lost on these diodes anyway? 5-10 Watts at full load?

Also aren't these switching events are happening with (50Hz) 20ms repetition times for just a duration of less than 100ns ? These are events with less than 0.001% duty ratio. Why is trr so important then?

I ve seen lots of examples using these diodes all over the place, some supplies don't even have a TO220 mount-cooled fast diode.






  • \$\begingroup\$ What is the "follow up" diode between, does the SMPS have active PFC? \$\endgroup\$ Nov 28, 2022 at 9:34
  • \$\begingroup\$ @TimWilliams By follow-up diode I meant the Fast-diode following the full bridge rectifier. It is like there is two stages of rectification (one full bridge "followed" by that half bridge with fast recovery diode). Follow-up is not the correct mainstream term. I don't know what do we call them or if there is a term for that... Also, yes, the SMPS have active PFC. \$\endgroup\$ Dec 7, 2022 at 21:43
  • \$\begingroup\$ Well, a "follow-up" doesn't make sense, as you've surmised, but that does describe the typical layout used with PFC: the diode bypasses the PFC inductor during startup (inrush), so the PFC doesn't risk accidentally sinking that current. The key is there's other stuff connected there (PFC inductor, MOSFET, fast diode) which is what's active in normal operation. Would be a bit of an oversight to leave those out of the path. But with them included, that explains that. \$\endgroup\$ Dec 7, 2022 at 23:49
  • \$\begingroup\$ @TimWilliams Are we sure we have a PFC inductor there? I am sure this board has active PFC, not passive PFC. Board backside: i.imgur.com/igaUpe9.jpeg board Frontside: i.imgur.com/7IAXUVg.jpeg \$\endgroup\$ Dec 10, 2022 at 20:36
  • \$\begingroup\$ Hah that looks awfully familiar, EVGA something or other? Looks like a smaller version of one I have. Yes, that's a PFC inductor, note the MOSFET and diode connected to it. \$\endgroup\$ Dec 10, 2022 at 21:03

1 Answer 1


There is some advantage to a slow diode, besides cost and voltage drop.

The slow recovery is also gradual, dissipating reactive energy as heat rather than snapping off and creating a sharp or ringing waveform that can conduct/radiate electromagnetic emissions (radio interference). The "bug" can be a feature!

The surge rating is slightly higher, more or less reflecting the lower VF, though there may be other reasons.

A few words about standards:

1Nnnn are (mostly) ancient JEDEC standards. In this earliest type system, parts were manufactured to target multiple types, and after testing and packaging, the same chip (process and masks) might be sold as many types. Multiple manufacturers sourced these parts, as demanded by major customers at the time -- military especially. You could be assured that any make of these parts will meet the common standard.

As such, these standards were often very lax. Notice how many specs are limiting values: minimums or maximums. 2N2222 for example only specifies minimum fT. How did you know you weren't going to get some random hotrod RF part "mis"labeled as one? That's the neat part, you don't. Probably this didn't happen very often because such a chip can be sold as something more profitable, but it did affect other types. The (originally) ponderously slow 2N3055 is a prime example: when manufacturers discontinued the original hometaxial process and migrated these parts to modern epitaxial lines, the fT went up by several times. Amplifiers built with long wires to the transistors, that formerly were normal and stable, became power oscillators!

These limits affect the 1N540x series as well. You're guaranteed to get something that meets the standard -- but how much it exceeds it (or what the variance is on min/max constrained values), you don't know.

We know that rectifier diodes are heavily optimized for low forward drop.

Do we?

It seems, with process improvements over the decades (mostly the earliest decades; this too is ancient history), the VF/VRRM ratio has been optimized well enough that meeting the basic spec is trivial.

Once the basic spec has been met, is there really any value in pushing further? You might assume "strictly better" means unconditionally better, but might there be more value in optimizing other parameters instead?

It turns out there is. In fact, an average 1N5408 likely breaks down over 1400V. It seems most manufacturers have decided that erring on the side of robustness is a higher priority than further reducing VF. Since mains surges are in the couple-kV range, this greatly increases reliability -- even when used without MOVs or other surge protection devices.

You would actually have to look at avalanche rectifiers to get that characteristic -- although even among them, you don't see it very often. Here's an example that does specify V(BR): Vishay BYW82. Even here, it seems likely they target the worst case type in the series; presumably avalanche current could be higher at lower V(BR) for applicable types, but they didn't design this standard that way. (I'm not sure of the history of the BYW and related series; it may be proprietary to a manufacturer, I don't know.)

Furthermore, notice how all parts in the family are specified with the same values. Interesting, huh? You may find a 1N5401 breaks down anywhere from 400 to 1800V! (I haven't seen any "standard" rectifiers below this range, myself. Checking: correction, have seen one 1N4001 at 363V. Well, close.)

So, for repair purposes, meet or exceed VF, VRRM, IF(AV), IFSM, and trr if it's a fast type. Substituting fast for GP, or substituting PN ↔ schottky, or excessive CJO, etc., may worsen EMI.

You could indeed substitute modern parts and likely improve efficiency, but a complete evaluation would have to be done to eke out every point of gain (e.g. adjusting or removing snubbers), and then EMI would have to be checked, and either filtering improved or compromises made to get it back under limits if exceeded.

And I emphasize EMC here, mainly because it's the least understood topic by EEs, and most likely to change with these kind of substitutions. In practical terms, it might matter very little. As far as I know, in most places you stand very little chance of being prosecuted by authorities, to whatever extent regional authorities have the right to do so in the first place. But it's still a practical matter, if nothing else, and a spooky one at that. Maybe you don't notice anything wrong, ever. Maybe you get a persistent buzzing sound in your audio equipment now. (Cheap audio equipment is especially sensitive to RF; the buzzing and blipping sounds of GSM telephone communications are likely familiar to many people.) Maybe there's increased noise in your lab environment (making ~mV level measurements on the oscilloscope suddenly awkward). Maybe your neighbor's garage door stops working. Etc. It's a spooky situation, it may seem arbitrary and random. And it can be hard to track down. EMI tends to conduct throughout its (wiring) environment, you might measure similar levels throughout the building. The cost is likely small (unlikely to be a problem, and when it is, it's likely more just an annoyance), but persistent and so it may add up over time; meanwhile, fixing it is no small feat, most people don't have the equipment to track down and evaluate such equipment. So it's best to avoid such issues in the first place.

  • \$\begingroup\$ So much information is embedded in this post. Amazing. I just learned that max reverse voltage quoted is not the breakdown voltage. I just did the replacements. I replaced 25ns TRR fast diode with a 60ns TRR one (MUR1560G). I checked the temps under a thermal camera, and some load but there is not much temp increase except the diode bridge. I understood that the the diode bridge+fastDiode also consumes 50-60% of the idle power of that ATX power supply. 11W idle power (~5W on the 12V 200mA cooling fan) . Without the fan blowing on top of them I measured their temperature reaching 55*C in idle. \$\endgroup\$ Dec 7, 2022 at 21:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.